Thin-film semiconductor device, display device including the same, and method of driving display device

ABSTRACT

A thin-film semiconductor device includes a temperature sensor formed of a thin-film semiconductor and sensing a temperature as current, and a current-voltage converter formed of a thin-film semiconductor and having temperature dependence in which its current-voltage characteristic is different from that of the temperature sensor. A temperature sensed by the temperature sensor is converted to a voltage by the current-voltage converter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin-film semiconductor device,driving circuitry thereof, and an apparatus incorporating the same andusing temperature information when operating them.

2. Description of the Related Art

Temperature sensors have been provided on the outside of display devicesto permit temperature-dependent correction of the driving waveform for aliquid crystal display. Thermistors, platinum resistance thermometers,thermocouples, temperature sensors with a pn junction diode usingbandgap, SAW (surface acoustic wave) temperature sensors,thermosensitive magnetic substances, and radiation thermometers using aninfrared ray are mainly used. Temperature sensors with a pn junctiondiode, which is inexpensive and has a high linearity to temperature, arebeing widely used. An attempt to substitute light measurement oftransmittance or reflectance for the temperature sensor is being made. Aphotodiode and a photoconductive cell are used for such lightmeasurement.

The prior art temperature sensors have some problems. When a temperaturesensor is on the outside of a display device, the indicated temperatureof the liquid crystal part is really only an estimated value, since thetemperature of the liquid crystal part held between the supportsubstrates is not directly measured. This will be described in detailusing FIG. 1. FIG. 1 shows a construction in which two temperaturesensors 83 are provided on the outside of a liquid crystal displaymanufactured by holding a liquid crystal 908 between glass substrates 10and 29. In the construction, the lower left side of the glass substrate29 is heated at 100° C., the glass substrate 10 side is at 20° C. andthe back surface of the upper right side of the glass substrate 29 is at35° C. Reflecting the temperature distribution, a temperaturedistribution of 25 to 65° C. occurs in the liquid crystal 908. On theother hand, the outputs of the temperature sensors are at 20° C. and 35°C. The temperature of the liquid crystal part cannot be correctlyidentified. The problem is significant especially in a sidelightconstruction in which the light source of a backlight is arranged at oneside. A temperature distribution is different in accordance with aportion of the apparatus in other constructions. Externalizing thetemperature sensor from the display makes it difficult to correctlymeasure a temperature when the liquid crystal is operated.

Including a temperature sensor in a display device has also beenconsidered. In JP2000-338518 (reference 1), a temperature sensing deviceformed on the same substrate as a thin-film transistor to drive a liquidcrystal is used. FIGS. 2A and 2B show equivalent circuits thereof. FIG.2A shows an equivalent circuit constructed of a thin-film transistor 4in which a gate electrode is short-circuited with a drain electrode or asource electrode. FIG. 2B shows an equivalent circuit constructed of athin-film diode 5. In reference 1, a constant-current source located onthe outside is connected to both ends of the temperature sensing deviceto sense a temperature. It is generally considered that using a currentsource formed on the same substrate as forms the temperature sensor caneliminate any noise problem. When an electric current is constant, avoltage applied to both ends of a diode depends on ambient temperature.This publication shows that a temperature can be sensed from thedrain-source voltage. The temperature sensor using a thin-film diodemanufactured in a liquid crystal display device measures a temperaturewhich is very close to the temperature of the liquid crystal itself ascompared with the temperature sensor outside the display. However, thecurrent source is outside of the device, which is susceptible to noisefrom external apparatus. The temperature sensor needs to sense a verysmall electric current of several to several tens of nanoamperes. Thus,lowered accuracy due to externalization of the current source cannot beavoided. Moreover, thin-film semiconductors represented by amorphoussilicon, polysilicon, and CG silicon cannot satisfactorily form the pnjunction part as compared with a semiconductor using bulk silicon. Thereference voltage is easily varied, and the sensed temperatures arevaried.

It is generally considered that forming a current source on the samesubstrate as that of a temperature sensor can solve any noise problem.However, when the current source and the temperature sensor are formedon the same substrate, they have equal current change to temperaturechange so as to cancel the change in each other. Therefore, it isdifficult to sense the temperature change.

Yannis Tsividis has reported in Yannis Tsividis, “Operation and Modelingof The MOS Transistor”, Second edition, WCB/McGraw-Hill, pp. 183-190that the gate voltage-drain current characteristic of transistors madeof bulk silicon exhibits different temperature dependence by a gatevoltage. As shown in FIG. 3, as an example, the temperature dependenceof a drain current of the transistor made of bulk silicon is hardly seennear a gate voltage of 0.9 V, about twice a threshold value of 0.5 V. Ina region lower than the gate voltage, the drain current is higher as thetemperature is increased. In a region higher than the gate voltage, thedrain current is higher as the temperature is decreased. In bulksilicon, a transistor as a temperature sensor and a transistor as aconstant-current source are manufactured on the same substrate. Theformer is driven in a gate voltage region having temperature dependence.The latter is driven by a gate voltage region having small temperaturedependence. In principle, temperature change can be sensed as a voltage.

On the other hand, a semiconductor layer used for a liquid crystaldisplay is of amorphous silicon, polysilicon, or CG silicon, not of bulksilicon. The threshold values are distributed in a wide range and cannotbe uniquely determined. Unlike transistors made of bulk silicon, it isdifficult to set a gate voltage value based on a threshold value.Temperature monitoring with high accuracy is hard.

In thin-film semiconductor devices, with any of the methods ofexternalizing a temperature sensor, of locating a temperature sensorinside and having a current-voltage converter outside, or of having botha temperature sensor and a constant-current source inside, it is stilldifficult to measure the temperature of a liquid crystal with sufficientaccuracy for controlling the liquid crystal.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a thin-filmsemiconductor device which can take out a temperature as a voltage.

Another object of the present invention is to provide a method ofdriving a thin-film semiconductor device.

A further object of the present invention is to provide an apparatuswhich incorporates a thin-film semiconductor device which can sense atemperature as a voltage, to perform control of the operation of theapparatus corresponding to temperature change. For instance, it is toprovide a liquid crystal display which can perform satisfactory imagereproduction over a wide temperature range by changing the drivingvoltage and the light source driving method of the liquid crystaldisplay in accordance with temperature.

A thin-film semiconductor device according to the present invention hasa temperature sensor made of a thin-film semiconductor and sensing atemperature as current, and a current-voltage converter made of athin-film semiconductor and having temperature dependence in which itscurrent-voltage characteristic is different from that of the temperaturesensor, wherein a temperature sensed by the temperature sensor isconverted to a voltage by the current-voltage converter.

The thus-constructed thin-film semiconductor device according to thepresent invention can exhibit the function of temperature monitoring byconverting a temperature-dependent current in the temperature sensor toa voltage by the current-voltage converter. According to the presentinvention, the temperature sensor and the current-voltage converter havedifferent temperature dependence. Despite that both the temperaturesensor and the current-voltage converter are made of the same thin-filmsemiconductor, this configuration can achieve a temperature-dependentvoltage value with sufficient accuracy. The present inventors have foundfor the first time that in the thin-film semiconductor device, gatevoltage regions in which the gate voltage-drain current characteristicis not dependent on temperature exist not only near the threshold valuebut also in a saturation region in which the drain current is saturated,and have made the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the problems of prior art temperaturemeasurement in a liquid crystal display provided with temperaturesensors outside the display;

FIG. 2A shows prior art temperature sensors, in which FIG. 2A shows adiode connected thin-film transistor and FIG. 2B shows a thin-filmdiode;

FIG. 3 is a diagram showing temperature dependence of drain voltage inchanging gate voltage in the transistor manufactured by bulk silicontechnology;

FIG. 4 is a diagram showing a thin-film semiconductor device of FirstEmbodiment of the present invention;

FIGS. 5A, 5B, and 5C are diagrams showing examples of a circuit of athin-film semiconductor device of First Embodiment of the presentinvention;

FIG. 6 is a diagram showing temperature dependence of drain voltage inchanging gate voltage in a thin-film transistor made by the polysilicontechnology of a thin-film semiconductor device of First Embodiment ofthe present invention;

FIG. 7 is a diagram showing temperature dependence of the relationbetween input voltage and output voltage of the temperature sensor madeby the polysilicon technology according to First Embodiment of thepresent invention;

FIG. 8 is a diagram showing the relation between temperature and outputvoltage of the temperature sensor at an input voltage of 5 V accordingto First Embodiment of the present invention;

FIG. 9 is a diagram showing the relation (a control voltage of 15 V)between temperature and output voltage of the thin-film semiconductordevice using a region six times (or more) larger than the thresholdvalue as the control voltage of the current-voltage converter 2;

FIG. 10 is a diagram showing the relation between the ratio of controlvoltage to threshold voltage and the change of output voltagecorresponding to temperature change by 5° C. at each temperature;

FIG. 11 is a diagram showing the relation among the ratio of controlvoltage to threshold voltage, gain, and linearity;

FIG. 12 is a functional block diagram showing feedback control of thedriving circuitry reference voltage of a liquid crystal display by thethin-film semiconductor device;

FIG. 13 is a functional block diagram showing control of the backlightof the liquid crystal display by the thin-film semiconductor device;

FIG. 14 is across-sectional view showing the cross-sectional structureof a planar type polysilicon TFT switch used in First Embodiment of thepresent invention;

FIG. 15 is a diagram showing a mask pattern for forming the thin-filmsemiconductor device according to First Embodiment of the presentinvention performing connection of polysilicon and a gate electrode viaa metal used in a drain and source electrode;

FIG. 16 is a cross-sectional view taken along line A-A′ of the thin-filmsemiconductor device manufactured in FIG. 15;

FIG. 17 is a diagram showing a mask pattern for forming the thin-filmsemiconductor device according to First Embodiment of the presentinvention directly performing connection of polysilicon and a gateelectrode;

FIG. 18 is a cross-sectional view taken along line B-B′ of the thin-filmsemiconductor device manufactured in FIG. 17;

FIG. 19 is a cross-sectional view of the thin-film semiconductor deviceusing a bottom-gated TFT;

FIGS. 20A and 20B are diagrams showing a method of manufacturing asource and drain region in the thin-film semiconductor device using thebottom-gated TFT;

FIG. 21 is a diagram showing temperature dependence of drain voltage inchanging gate voltage in a transistor manufactured by the partiallydepleted SOI technology of the thin-film semiconductor device;

FIG. 22 is a diagram showing temperature dependence of the relationbetween input voltage and output voltage of the temperature sensor bythe partially depleted SOI technology according to Second Embodiment ofthe present invention;

FIG. 23 is a diagram showing temperature dependence of drain voltage inchanging gate voltage in a transistor manufactured by the fully depletedSOI technology of the thin-film semiconductor device of the thirdembodiment of the present invention;

FIG. 24 is a diagram showing temperature dependence of the relationbetween input voltage and output voltage of the temperature sensor bythe fully depleted SOI technology according to Third Embodiment of thepresent invention;

FIG. 25 is a diagram showing the circuitry configuration of anamplifying part according to Fourth Embodiment of the present invention;

FIG. 26 is a diagram showing temperature dependence of the relationbetween bias voltage and output voltage of the amplifying part accordingto Fourth Embodiment of the present invention;

FIG. 27 is a diagram showing the relation between temperature and outputvoltage of the amplifying part at a bias voltage of the amplifying partof 9.75 V according to Fourth Embodiment of the present invention;

FIG. 28 shows a top view of a exemplary configuration of a RF-ID device;

FIG. 29 shows a top of view of an exemplary configuration of a biochip;

FIG. 30 is a functional structure diagram of the liquid crystal displayusing an overdrive system;

FIG. 31 is a functional block diagram of the liquid crystal displayusing the overdrive system;

FIG. 32 is a functional block diagram of overdrive circuitry using acapacitance predicting LUT (lookup table);

FIG. 33 is a functional structure diagram applying feedback control bythe thin-film semiconductor device to the liquid crystal display of thefield sequential color;

FIG. 34 is a system overall diagram of a liquid crystal display of thefield sequential color type; and

FIG. 35 is a diagram showing the ratio of output voltage at eachtemperature to output voltage at 20° C. with temperatures indicated onthe horizontal axis in First Embodiment and Comparative Example 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In typical bulk silicon technology, a temperature-independent region islimited to near a voltage twice the threshold value. On the other hand,the threshold value of thin-film semiconductor transistors varieswidely. When using a region near the threshold value as the operatingvoltage of the current-voltage converter, a slight change of the voltagecan cause temperature dependence. The temperature-independentcharacteristics of the saturation region are not changed when thevoltage is slightly changed. The voltage of the saturation region isused as the operating voltage of the current-voltage converter and theregion having temperature dependence is used as the operating voltage ofthe temperature sensor. Current change caused in the temperature sensorcan be sensed as a voltage by the current-voltage converter.

Specifically, in the thin-film semiconductor device according toembodiments of the present invention, the current-voltage converter is athin-film transistor having a thin-film semiconductor layer, a gateelectrode applying a gate voltage to the thin-film semiconductor layer,and a drain electrode and a source electrode conducting an electriccurrent to the thin-film semiconductor layer. To sense temperaturechange as an electric current, the gate voltage is set to a saturationregion in which a drain current is saturated. Preferably, the gatevoltage is larger than three times the threshold voltage of thethin-film transistor. As the temperature sensor, a diode connectedthin-film transistor or a pn junction diode can be used.

According to embodiments of the present invention, the thin-filmsemiconductor device has on a substrate having a thin-film transistorarray to drive pixels, a temperature sensor and a current-voltageconverter connected to the temperature sensor and having temperaturedependence different from that of the temperature sensor. It is possibleto obtain a thin-film semiconductor device capable of sensing an outputcurrent changed depending on a temperature sensed by the temperaturesensor as a voltage with high accuracy without canceling by thetemperature characteristic of the current-voltage converter. Using thiscan obtain a liquid crystal display which can perform satisfactory imagereproduction over a wide temperature range.

First Embodiment

1. Basic Configuration

As shown in FIG. 4, this embodiment of the present invention has afundamental feature of having a temperature sensor 1 and acurrent-voltage converter 2 on the same substrate. Current change due totemperature change of the substrate sensed by the temperature sensor 1is converted to a voltage by the current-voltage converter 2. Thetemperature change of the substrate can be observed as voltage change.

FIG. 5A is a schematic diagram showing this. The temperature sensor 1and the current-voltage converter 2 are each made of a thin-filmtransistor (TFT) and are formed on the pixel driving TFT arraysubstrate. The gate electrode of the temperature sensor 1 isshort-circuited with the source electrode and is connected to the drainelectrode of the current-voltage converter 2. A constant negative dcvoltage is applied to the power source electrode 54 of the temperaturesensor 1. A constant input voltage is applied to the input electrode 52of the current-voltage converter 2. Then an electric current dependenton substrate temperature flows in the temperature sensor 1. The electriccurrent can be monitored at an output electrode 53 as a voltagedetermined by a control voltage in the control electrode 51.

As shown in FIG. 5B, using the pn junction diode of a thin-filmsemiconductor as the temperature sensor can obtain the same function. Asshown in FIG. 5C, without short-circuiting the gate electrode and thesource electrode of the temperature sensor 1, a voltage is independentlyapplied to the gate electrode of the temperature sensor 1 and the gateelectrode of the current-voltage converter 2 to realize the presentinvention. By way of example, the size of the thin-film transistor has agate length of 4.mu.m and a gate width of 4.mu.m. Polysilicon is used asthe semiconductor layer.

FIG. 6 shows an example of the gate voltage dependence of the draincurrent of such thin-film transistor. The horizontal axis indicates gatevoltage and the vertical axis indicates drain current in logarithmicplot. The thin-film transistor has a threshold voltage of 0.9 V. Whenchanging the temperature to −40° C., 20° C., and 80° C., a region inwhich the drain current is independent of temperature exists at gatevoltages of 0.3 V and 12 V. The drain current decreases with increasingtemperature in a region of 0.3 to 12 V. The drain current increases withincreasing temperature in a region above 12 V and a region below 0.3 V.

2. Characteristic when Using a Control Voltage Giving a Drain CurrentIndependent of Temperature

A region independent of temperature is used for a gate voltage drivingthe thin-film transistor of the current-voltage converter 2 (called acontrol voltage). 0.3 V is near the threshold value so that thecharacteristic cannot be stable. Thus, the control voltage is set tonear 12 V of the saturation region.

FIG. 7 shows an input/output voltage characteristic when the controlvoltage is 10 V in FIG. 5A. Temperature dependence can be obtained inthe input voltage range of 2 to 8V. The temperature sensitivity near 5 Vis highest. FIG. 8 shows change in output voltage to temperature whenfixing an input voltage to 5 V. The output voltage exhibits asatisfactory linearity to temperature. The voltage rise value per kelvinis 1.5 mV. The threshold value of the TFT made by the polysilicontechnology is varied greatly. This embodiment uses the gate voltagesufficiently larger than the threshold value and is hardly susceptibleto variation in the threshold value. In the configuration of FIG. 5B,almost the same result can be obtained. In the configuration of FIG. 5C,the control voltage is 10 V and the gate voltage of the temperaturesensor 1 is 3 V where the temperature dependence is large in FIG. 6. Inthis case, almost the same result can be obtained.

3. Characteristic when Using a Control Voltage Giving a Drain CurrentWhose Temperature Dependence is Opposite to that of the TemperatureSensor

As the control voltage of the current-voltage converter 2, a region sixor more times the threshold value where the temperature dependence isopposite to that of the temperature sensor 1 is used for temperaturesensing. By way of example, FIG. 9 shows the relation betweentemperature and output voltage when setting the control voltage to 15 V,the gate voltage of the temperature sensor to near 3 V, and the inputvoltage to 5 V in the configuration of FIG. 5C. The temperaturedependence of the current-voltage converter 2 is opposite to thetemperature dependence of the temperature sensor 1, as shown in FIG. 6.Temperature sensing having temperature dependence opposite to that ofthe temperature sensor 1 is thereby enabled. In this case, the outputvoltage also exhibits a satisfactory linearity to temperature. Thevoltage rise value per kelvin is 3.0 mV. Sensitivity can be achievedthat is about twice that obtained when using the control voltage givingthe temperature-independent drain current in the previous section.

4. Optimizing a Control Voltage

To determine an optimum control voltage as the temperature sensor,control voltage dependence of the temperature sensitivity is inspected.Change in output voltage to temperature change by 5° C. is measured bychanging the ratio of control voltage to threshold voltage. FIG. 10shows the result. The horizontal axis indicates the ratio of the controlvoltage to the threshold voltage. The vertical axis indicates change inoutput voltage when temperature is changed by 5° C. The condition inwhich the ratio of the control voltage to the threshold voltage is 2.0is the condition of the later-described Comparative Example 1. Whenchanging the temperature in this condition, there is hardly any voltagechange. Voltage change occurs slightly at the ratio of 2.5; however,that value is too small for optimum use as the temperature sensor. Atthe ratio of 3.0, the average value of the voltage change is about 0.5mV/K and can be practical as the temperature sensor. At the ratio of3.5, the average value of the voltage change is about 1.1 mV/K. At theratio of 4.0, the average value of the voltage change is about 1.5 mV/K.When the ratio of the control voltage to the threshold voltage is 3 orabove, a preferred function as the temperature sensor can be obtained.Measurement enlarging the range changing the ratio of the controlvoltage to the threshold voltage is made for regression analysisprocessing. The relation between the ratio of voltages and thetemperature sensitivity can be generalized in a wider range. In themeasurement, the average ratio of output voltage change to temperaturechange is defined as a gain in the measured temperature range. The gaincorresponds to a regression coefficient when temperature is set as thehorizontal axis and output voltage is set as the vertical axis so thatthe regression analysis is applied to the obtained characteristic bycollinear approximation. A determination coefficient (called an R2value) in the regression analysis exhibits a dispersion of measurementvalues around the regression line. The determination coefficientexhibits a linearity of the output voltage to the temperature. The valueis in the range of 0 to 1. As it is close to 1, the linearity is morepreferred.

FIG. 11 shows the relation between the gain and linearity when changingthe ratio of the control voltage to the threshold voltage to 0.5, 1.0,2.0, 2.5, 3.0, 3.5, 4.0, 6.0, and 10.0. When the ratio of the controlvoltage to the threshold voltage is 2.0, the output voltage value isalmost constant. In this case, the regression analysis in whichtemperature is indicated by the horizontal axis cannot determine acorrect determination coefficient. Thus, the linearity at the ratio of2.0 is not plotted. As is understood from the drawing, a region in whichthe ratio of the control voltage to the threshold voltage is smallerthan 1.0 obtains a characteristic having a very high linearity. As inthe ratio of 0.5, a condition exists in which a certain amount of gainis obtained. The region can be used as the temperature sensor. Asdescribed above, the region is significantly susceptible to thethreshold voltage. When using it by the polysilicon technology, ameasured error becomes large. At the ratio of 2.5, the gain is low andthe linearity is not so satisfactory. When the ratio of the controlvoltage to the threshold voltage is 3.0 or above, the gain is 0.5 mV orabove per kelvin and also the linearity above 95% is obtained. At theratio of 3.0 or above, the gain and linearity both exhibit satisfactoryvalues. The region can be used as the temperature sensor.

The control voltage need not be applied continuously but may instead beapplied intermittently in the cycle where temperature information isnecessary. By this intermittent access, lower power consumption can beobtained. In the thin-film semiconductor device according to the presentinvention, when applying the control voltage, an electric current flowsbetween the input electrode and the negative power source electrode.Temperature change of the electric current is measured to obtaintemperature information. In place of the control voltage, a voltage inwhich the transistor of the current-voltage converter of FIG. 5A isbrought to the off state is applied. Then, the electric current in thetransistor of the current-voltage converter is almost zero. At the sametime, the electric current in the temperature sensor does not flow sothat the current consumed is almost zero. When temperature change of anobject is about 1° C. per second, only one measurement per second maygive sufficient temperature information. When measuring temperature in a10-millisecond period once per second, intermittent application of thecontrol voltage reduces the consumption current to 1/100. Leakagecurrent in the off state and current consumption caused by changing avoltage applied to the control electrode cause the actual currentconsumption to be about 1/25.

5. Driving Control by the Thin-Film Semiconductor Device

FIG. 12 shows an example of a functional block feeding back temperatureinformation obtained by the thin-film semiconductor device to thereference voltage of liquid crystal display driving circuitry. Theliquid crystal display area is driven by signal electrode circuitry 31and scanning electrode circuitry 32. The operation of these iscontrolled by control circuitry 36. The amplitude of signals produced bythe signal electrode driving circuitry is determined by a referencevoltage produced by reference voltage circuitry 35. A thin-filmsemiconductor device 3 of the present invention is disposed on theliquid crystal display area. The signal amplitude of the referencevoltage 35 is controlled by an output signal from the thin-filmsemiconductor device 3. The amplitude of a signal voltage actuallyapplied to the liquid crystal can be adjusted in accordance withtemperature to obtain image quality regardless of temperature. Thereference voltage adjusted by the temperature information is appliedfrom the reference voltage circuitry 35 to the liquid crystal displayarea and the signal electrode driving circuitry 31. The amplitude of asignal voltage actually applied to the liquid crystal can be adjusted inaccordance with temperature.

As shown in FIG. 13, the temperature information outputted from thethin-film semiconductor device 3 can be fed back to the backlight. Tobrighten the screen, in the liquid crystal display the display area isilluminated with light emitted from a light source 33. The brightness ofthe light source 33 is controlled by light source driving circuitry 34driven by the control circuitry 36. The output voltage of the thin-filmsemiconductor device 3 disposed on the liquid crystal display area isfed back to the light source driving circuitry 34 to adjust the lightsource brightness in accordance with temperature change. When thetemperature is lowered and the light efficiency of the liquid crystaldisplay area is reduced, the configuration controls the brightness ofthe backlight to increase in order to reduce the influence of thelowered temperature on the image reproduction.

6. TFT Manufacturing Method

In this embodiment, a TFT manufacturing method by the polysilicontechnology which is used most widely will be described. Here, thepolysilicon technology refers to a technology forming a polysilicon thinfilm having a polycrystalline structure on an insulator film on asubstrate.

(1) Basic Manufacturing Method

There are two TFT structures of the top-gated and the bottom-gatedtypes. Application of the top-gated TFT will be described using FIG. 14.A silicon oxide film 28-1 is formed on a glass substrate 29, then anamorphous silicon film is formed. An excimer laser is used in annealingto modify the amorphous silicon to polysilicon 27. After patterning it,the silicon oxide film 28-2 of 10 nm is formed. A photoresist ispatterned to be slightly larger than the gate shape (to form LDD regions23 and 24 later). Phosphorous ions are doped to form a source region anda drain region. After growing the silicon oxide film 28-2 as a gateoxide film, the amorphous silicon and tungsten silicide (WSi) as a gateelectrode are grown. Patterning those with a photoresist mask forms thegate electrode. With the patterned photoresist as a mask, phosphorousions are doped only in necessary regions to form the LDD regions 23 and24.

The silicon oxide film 28-3 is grown. Via holes are formed. Aluminum andtitanium are formed by sputtering, then patterned to form a sourceelectrode 26 and a drain electrode 25. The silicon nitride film 21 isformed on the entire surface, then a contact hole is formed. An ITO filmis formed on the entire surface for patterning to form a transparentpixel electrode 22.

The planar type TFT pixel switch as shown in FIG. 14 is manufactured toform a TFT array. The pixel array with the TFT switches and the scanningcircuitry are formed on the glass substrate. Arraying can be made by theprior art manufacturing method. In FIG. 14, the TFT is formed by thepolycrystallization of amorphous silicon. The TFT may be formed by amethod of improving the grain size of polysilicon with laser irradiationafter growing. Other than the excimer laser, continuous-wave (CW) lasermay be used.

(2) Thin-Film Semiconductor Device Manufacturing Method

The thin-film semiconductor device according to the present inventioncan be manufactured based on the processes described in (1). FIG. 15shows an example of a mask layout diagram used here. FIG. 16 shows across-sectional view of the device manufactured using the mask along theline A-A′ of FIG. 15. In this example, the gate electrode and the drainelectrode of the temperature sensor and the source electrode of thecurrent-voltage converter are connected via the metal of the outputelectrode 53 and are equivalent to a diode. FIG. 17 shows an example ofanother mask layout. FIG. 18 shows a cross-sectional view of the devicemanufactured using the mask along the line B-B′. In this example, thegate electrode and the drain electrode of the temperature sensor and thesource electrode of the current-voltage converter are connected via apolysilicon film, not the metal material of the output electrode. Inthis structure, only contact between the gate electrode and the dopedpolysilicon film exists in the diode connection part.

The top-gated TFT in which the gate electrode is above the thin-filmsemiconductor layer is described above. The present invention can berealized by the bottom-gated TFT in which the gate electrode in belowthe thin-film semiconductor layer. A method for manufacturing thethin-film semiconductor device according to the present invention whenusing the bottom-gated TFT will be described.

FIG. 19 is a cross-sectional view of the device of the embodimentaccording to the present invention of the bottom-gated TFT. It is thesame as the layout of the thin-film semiconductor device in which thegate electrode and the drain electrode of the temperature sensor and thesource electrode of the current-voltage converter are connected via themetal of the output electrode described in FIGS. 15 and 16 except thatthe gate electrode is the lowest layer. FIGS. 20A and 20B show a methodfor manufacturing source and drain regions in the manufacturing methodof FIG. 19. Silicon oxide, not shown, is deposited on the glasssubstrate 29 and chromium (Cr) is formed thereon as a gate electrode 30.The silicon nitride film or the silicon oxide film 28 is formed as agate insulator film by the PE-CVD method and then amorphous silicon 12is deposited. The pattern of a photoresist 14 is formed as a doping maskon the channel region of the amorphous silicon 12 to dope impurities(phosphorus or boron) into the source and drain region by the ion dopingmethod (FIG. 20A). The photoresist 14 is removed, followed by thermaltreatment at 400° C. for 90 minutes in vacuum to reduce the hydrogenconcentration of the amorphous silicon film 12, whereupon the film 12 isirradiated with an excimer laser to crystallize the amorphous siliconfilm 12 (FIG. 20B) The impurities doped region is activatedsimultaneously with crystallization. After forming the source and drainelectrodes, the silicon oxide film is deposited and hydrogenated for 90minutes in a hydrogen plasma atmosphere at 300° C. to reduce danglingbond existing in the grain boundary of the poly-Si thin film.

According to First Embodiment of the present invention, the voltageapplied as the control voltage is set to an appropriate range. Currentchange due to temperature change sensed by the temperature sensor 1 canbe sensed as voltage change by the current-voltage converter 2. Thecontrol voltage is set to the region in which the drain current does notcause temperature dependence or the region whose temperature dependenceis different from the temperature dependence of the temperature sensor.The current-voltage converter 2 and the temperature sensor 1 displayindependent temperature characteristics, and so a satisfactorytemperature sensor can be achieved.

Second Embodiment

This embodiment is different from the first embodiment in that the TFTis formed by the partially depleted SOI technology, not the polysilicontechnology.

The SOI technology refers to a technology forming a single crystalsilicon thin film on an insulator film formed on a substrate. The SOItechnology is classified as the partially depleted SOI technology andthe fully depleted SOI technology. The difference between both is thedegree of depletion in a silicon film and specifically is divided basedon the thickness of the silicon film. The process when the thickness ofthe silicon film is twice or more the thickness of the thickestdepletion layer is called partially depleted SOI technology. Thegreatest thickness of the depletion layer depends on the amount ofimpurities and Fermi potential level and is different for each of thedevice technologies. The silicon film thicknesses of the partiallydepleted and the fully depleted are different for each of the devicetechnologies. Generally, the film thickness less than 50 nm is calledthe fully depleted and the film thickness of 100 to 200 nm (or above) iscalled the partially depleted. Here, they are defined in the same way.

As compared with the bulk silicon technology, the SOI technologyincludes the following features:

(1) allowing low voltage and fast operation because of a small junctioncapacitance;

(2) offering a high resistance to a radiation; and

(3) densely integrating the circuits with less cross talk.

For the substrates for the SOI technology, an SIMOX substrate of oxygenion implantation, a UNIBOND (Smart Cut) substrate by hydrogen ions,bonding and separation, and an ELTRAN substrate by bonding using aporous silicon substrate and water jet separation are currentlymanufactured.

In the partially depleted SOI, the process of manufacturing the deviceis the same as the typical bulk silicon process except for using adifferent silicon substrate. A bonded substrate is often used for thepartially depleted SOI. The thickness of the oxide film of the bondedsubstrate is controlled with high accuracy. For the silicon portion onwhich polishing is mechanically performed and the film thickness iseasily varied, however the film thickness of the partially depleted SOIis great so that the influence of the varied film thickness is less. Forthese reasons, a bonded substrate is often used.

The process when the thickness of the silicon film is less than thegreatest thickness of the depletion layer is called the fully depletedSOI technology. This will be described in the third embodiment.

FIG. 21 shows an example of the drain current-gate voltage dependence ofthe TFT manufactured by the partially depleted SOI technology. Themobility of the semiconductor layer of the SOI technology is nearly tentimes higher than that of the polysilicon technology of FIG. 6. There isno trap phenomenon in the grain boundary as seen in polysilicontechnology. The LDD structure is not used in this embodiment. The draincurrent is increased by nearly two orders of magnitude relative to thatof FIG. 6. As described above, the horizontal axis indicates gatevoltage, the vertical axis indicates drain current in logarithmic plot.When changing the temperature to −40° C., 20° C., and 80° C., a regionis identified in which the drain current is temperature-independent nearthe gate voltage of 1.9 V and near the gate voltage of 12 V.

FIG. 22 shows the input voltage-output voltage characteristic in thethin-film semiconductor device of the structure of FIG. 5A when thecontrol voltage is 12 V. As compared with the polysilicon technology,the temperature sensitivity is rather low. The voltage rise value perkelvin is 0.5 mV. The partially depleted SOI technology can neverthelessrealize an acceptable temperature sensor according to the presentinvention.

Third Embodiment

This embodiment is different from First and Second Embodiments in thatthe TFT is formed by fully depleted SOI technology, not polysilicontechnology. The fully depleted SOI technology is SOI technology whereinthe thickness of the silicon film is less than the greatest thickness ofthe depletion layer. The thickness is generally 50 nm or less. Thecurrent-voltage converter adjusts the voltage applied to the gateelectrode in the substrate direction (hereinafter, called a back gatevoltage) by the fully depleted SOI technology so as to allow in a dummymanner the bottom part of the body region to be undepleted. Theelectrode applying the back gate voltage is called a back gateelectrode. This structure can allow the back gate side of the depletedchannel in the fully depleted SOI technology to be undepleted.

FIG. 23 shows an example the drain current-gate voltage dependence ofthe TFT manufactured by the fully depleted SOI technology. A draincurrent higher than that of the polysilicon technology of FIG. 6 isobtained. As described above, the horizontal axis indicates gate voltageand the vertical axis indicates drain current in a logarithmic plot.When changing the temperature to −40° C., 20° C., and 80° C., a regionexists in which the drain current is temperature-independent near thegate voltage of 1.9 V and near the gate voltage of 12 V. Almost the sameresult as that of the partially depleted SOI technology can be obtained.FIG. 24 shows the input voltage-output voltage characteristic in thethin-film semiconductor device of the structure of FIG. 5A when thecontrol voltage is 12 V. The voltage rise value per kelvin is 0.6 mV. Atemperature sensitivity is slightly higher than that of the partiallydepleted SOI technology. The sensitivity is lower than that of thepolysilicon technology; however, the fully depleted SOI technology cannevertheless realize an acceptable thin-film semiconductor deviceaccording to the present invention.

As compared with Second and Third Embodiments, the First Embodiment withpolysilicon technology using the LDD structure is considered to be mostpreferred as the temperature sensor for the following reasons: 1.temperature sensitivity higher than that of the SOI technology; and 2.obtaining a temperature-independent characteristic in a lower currentregion to offer lower power consumption. Whereas circuitry usingpolysilicon technology conventionally have a problem of variation in thethreshold value, the present invention uses a region three or more timesthe threshold voltage and is relatively insusceptible to the thresholdvalue.

Fourth Embodiment

This embodiment connects the voltage output of the thin-filmsemiconductor device to an amplifier according to the first embodiment.

FIG. 25 shows the circuitry configuration of a manufactured amplifyingcomponent. The amplifying component has an operational transconductanceamplifier (OTA) configuration using a current mirror circuit and adifferential input circuit. In this embodiment, basic amplifyingcircuitry using a differential pair will be described. Other amplifyingcircuitry may be used.

The amplifying component of FIG. 25 uses seven transistors. All thetransistors have a gate length of 4 microns. Transistors T1, T2, T5, andT6 are p channel thin-film transistors. Transistors T3, T4, and T7 are nchannel thin-film transistors. The gate width of T1 and T2 is 7 microns.The gate width of T3 and T4 is 11 microns. The gate width of T5 is 22microns. The gate width of T6 and T7 is 75 microns. Compensationcapacitance Cf is 35 femto-farads (35 fF). The compensation capacitanceis provided by metal of the same layer as the gate electrode line andthe data electrode line. The film thickness of an oxide filmtherebetween is 4000 angstroms. The size of the electrode of thecapacitor is 65 microns by 6 microns. When connecting a load of 1picofarad (1 pF), the amplifying component can obtain a gain of 40 dB(decibels).

A differential input terminal 63 of the amplifying component in FIG. 25and an output electrode in FIGS. 5A, 5B or 5C are connected to constructthe thin-film semiconductor device. As for the temperature sensor 1 andthe current-voltage converter 2, a gate length is 4 microns, and a gatewidth is 12 microns. 10 V is applied to a positive source voltage line61 shown in FIG. 25 and a negative source voltage line 62 is grounded.

The characteristic of the thin-film semiconductor device is changeddepending on the magnitude of a bias voltage applied to the transistorsT5 and T6 corresponding to the constant current source. FIG. 26 showsthe result obtained by measuring the output voltage of the amplifyingcomponent of the thin-film semiconductor device when changing the biasvoltage. When changing the temperature to −40° C., 20° C., and 80° C.,the temperature dependence of the output voltage is found to be mostsignificant near the bias voltage of 9.8 V. Based on that result, thetemperature dependence of the output voltage of an output terminal atthe bias voltage of 9.75 V is measured. FIG. 27 shows the result. Thetemperature is changed from −40° C. to 80° C. by 5° C. increments. Asatisfactory linearity to temperature can be obtained. The voltage risevalue per kelvin is about 8 millivolts. In this way, an amplitude outputabout five times larger than that of the first embodiment that lacks theamplifying component can be obtained.

Fifth Embodiment

The fifth embodiment of the present invention is an RF-ID deviceperforming identification using a radio frequency and incorporates athin-film semiconductor device of the present invention. FIG. 28 shows atop view of an exemplary configuration of a RF-ID device. The RF-IDdevice includes IC 71 and an antenna 72 disposed on a substrate 73.There are various communication methods and frequency bandwidths of theradio frequency identification (RF-ID) device. By way of example, thereare a microwave method using a microwave, an electromagnetic inductionmethod using a band of several tens of kilohertz to several tens ofmegahertz, and an electromagnetic coupling method using a band ofseveral hundreds of kilohertz. In the present invention, the thin-filmsemiconductor device 3 is incorporated into the RF-ID device. Thetemperature-dependent variation in the operation of the device can becompensated in the RF-ID device. The temperature of the RF-ID device oran object provided with the RF-ID device can be monitored externally ina non-contact manner.

Sixth Embodiment

The sixth embodiment of the present invention is a biochip or a DNA chiphaving a thin-film semiconductor device of the present invention. FIG.29 shows a top view of an exemplary configuration of a biochip. Biochipsare broadly classified into an optical type and an electronic type. Inthe optical type biochip, a DNA fragment (probe) obtained from a knowngene is correctly positioned in a predetermined position to bechemically coupled to the substrate. The corresponding portions in anevaluated sample are coupled based on a lock and key relationship andlight is emitted by marker means such as luminescent dye for detectionby CCD. In the electronic type biochip, a substance to be inspected ismarked by an enzyme and is separated into two electrically activeportions so that a generated electric current can be detected by asensor having a gold electrode. The concentration of the targetsubstance is identified from the change of the electric current withtime. The polysilicon technology can realize the respective optical andelectronic types at low cost and can also realize a biochip mergingboth. When forming the thin-film semiconductor device 3 of the presentinvention on the biochip, temperatures when analyzing the manufacturedbiochip can be precisely monitored to increase the analysis accuracy.With the provision of the thin-film semiconductor device of the presentinvention, the temperature of a reacted portion can be preciselyidentified. Data useful for observing a living body can be obtained.

The DNA chip is often considered to be a kind of biochip. However, sincethe market this type of product is large, it may be classified as itsown product type. For most of the DNA chips, it is desirable to use aglass substrate such as slide glass rather then a silicon substrate dueto the necessity of fluorescent analysis. Typically, the DNA chipsubsequently synthesizes probes on the substrate using the lithographytechnology, which is very expensive. Using the glass substrate, whenperforming electric inspection, a special wiring technology isnecessary. It is difficult to add a circuit. In application of thepolysilicon technology, the glass substrate and the plastic substratecan be used, which can respond to fluorescent analysis. Wiring andcircuitry can be inserted between the probe and the glass substrate tomake the performance of the DNA chip high. The probe can be manufacturedat low cost by applying the ink jet technology (or the bubble jet(trademark) technology) for printing a DNA ink solution or spotting by amicro ceramics pump. This can provide a very high performance DNA chipby the polysilicon technology having electric circuitry at a cost lessthan 1/10 of the prior art cost. Application of the thin-filmsemiconductor device of the present invention to the biochip canprecisely monitor temperatures at reaction of DNA analysis for improvingthe accuracy of the DNA analysis.

Seventh Embodiment

In this embodiment, the thin-film semiconductor device of the presentinvention is applied to overdrive driving of the liquid crystal display.

FIG. 30 shows a functional block diagram applying the present inventionto the overdrive driving in the liquid crystal display. The basicconfiguration is the same as FIG. 13 described in the First Embodimentexcept that overdrive circuitry is connected to the control circuitry.The overdrive applies a voltage higher than that of normal driving tothe liquid crystal to increase a field intensity for promoting statechange, which is effective for driving the liquid crystal display fast.In this example, feed back to the light source is performed. FIG. 31shows an example of the functional block in the overdrive circuitry ofFIG. 30. In FIG. 31, the overdrive circuitry stores an input signal in aframe buffer 38, reads the signal of the immediately preceding framefrom the frame buffer 38, and compares it with the current input signal.Using the compared result, a video signal level after overdrive is readfrom a lookup table (LUT) 39 to determine a voltage to be actuallyapplied for outputting it as an output signal. In FIG. 30, correctionbased on temperature information using the thin-film semiconductordevice 3 is inputted to both the reference voltage circuitry 35 andoverdrive circuitry 37 for performing voltage correction in accordancewith temperature. The temperature information from the thin-filmsemiconductor device 3 is fed back to the light source driving circuitry34 driving the light source 33 for adjusting the brightness of the lightsource and light-emitting timing. In this embodiment, as the thin-filmsemiconductor device 3, the control voltage of the current-voltageconverter is 10 V and the input voltage thereof is 5 V.

When performing capacitance prediction in the overdrive method, in placeof the overdrive circuitry of FIG. 31, the overdrive circuitry of FIG.32 is used. The circuitry determines the predicted capacitance of thenext frame using a capacitance predicting LUT (lookup table) 44 based onan input signal and the capacitance of each pixel of the immediatelypreceding frame read from a capacitance frame buffer 45. The predictedcapacitance is stored in the capacitance frame buffer 45. A voltage LUT(lookup table) 43 compares it with the input signal and determines asignal to be actually applied for outputting it as an output signal.Using this circuitry, when a desired brightness cannot be obtained sincethe response of the liquid crystal is slow, the applied voltage can beadjusted so as to reach a necessary liquid crystal capacitancecorresponding to a desired brightness within a desired time. Overdrivedriving can be thus realized.

The response of the liquid crystal is influenced by temperature. Thevoltage LUT is prepared for each representative temperature to selectthe voltage LUT in accordance with a measured temperature. Voltagecorrection in accordance with temperature in the overdrive circuitry isperformed by this structure.

Eighth Embodiment

An example applying the present invention to driving of the liquidcrystal display of the field sequential color system (color timedivision) is shown. In the field sequential color system, the availableresponse time of the liquid crystals is very short. The influence of thelowered response speed of the liquid crystal when the temperature islowered is significant. In this embodiment, temperatures are monitoredby the thin-film semiconductor device to increase the overdrive voltageand the brightness of the backlight at low temperature for obtainingsatisfactory image not dependent on temperature.

FIG. 33 shows an example of a functional block when giving feedbackbased on temperature information in the liquid crystal display of thefield sequential system. The basic construction is the same as that ofFIG. 13 described in the First Embodiment except that field sequentialdriving circuitry 42 is connected to the control circuitry to use afield sequential light source 40 as a light source.

In the field sequential color system, a displayed image is divided intosome/several color sub images which are then displayed in sequentialmanner so that the light source of the same color as the sub image isilluminated in synchronization therewith. As the field sequential lightsource 40, a LED, an EL, or a cold cathode tube having plural wavelengthbands can be used. Field sequential light source driving circuitry 41 isdriven by the control circuitry 36 and has a function of switching thecolors of the field sequential light source 40 at high speed.

FIG. 34 shows an example of a field sequential display system in moredetail. Image data 110 is processed by a field sequential controller IC103 to be converted to image data for each of the colors red, blue, andgreen. The field sequential controller IC 103 has therein the controlcircuitry 36, a pulse generator 104, and high-speed frame memory 106.The image data 110 is once stored in the fast frame memory to be splitinto each of the colors. A LED control signal 108 driving a LED 101 isproduced from the control circuitry 36 based on information on thecolors. At the same time, the control circuitry 36 drives the pulsegenerator 104 to produce a driving pulse 109. The image data 110 isinputted via a digital analog converter (DAC) 102 to an LCD 100. The LCD100 receives the driving pulse 109 to illuminate the image of eachcolor. Ordinary image data is processed by the field sequentialcontroller IC 103 incorporating the control circuitry 36, the pulsegenerator 104, and the high-speed frame memory 106 to be converted toimage data for each of the colors of red, blue, and green. The imagedata is inputted via the DAC 102 to the LCD (liquid crystal displaypanel) 100. The scanning circuitry in the LCD 100 is controlled by thedriving pulse from the pulse generator of the controller IC. Using theLED 101 of three colors as the light source, the LED is controlled bythe LED control signal 108 from the controller IC.

FIG. 33 shows the field sequential controller IC 103 of FIG. 34 as twofunctional blocks of the control circuitry 36 and the field sequentialdriving circuitry 42. In the field sequential system, the colors of thelight source are switched at high speed to realize color display. Acolor filter is unnecessary for the liquid crystal display part. Colorswitching, illumination timing, and brightness are processed based on asignal of the control circuitry 36 in the field sequential drivingcircuitry 41. Like the overdrive system, temperature information of thethin-film semiconductor device 3 is fed back to the reference voltagecircuitry 35, the field sequential driving circuitry 42, and the fieldsequential light source driving circuitry 41. Constant image quality notdependent on temperature can be maintained. In this embodiment, as thethin-film semiconductor device 3, the control voltage of the TFT of thecurrent-voltage converter is 10 V and the input voltage of the TFT ofthe current-voltage converter is 5 V.

It is found that temperature control can lower brightness change andtemperature-dependent color that are seen in the prior art, thereby toimprove image quality.

Comparative Example

FIG. 35 shows the temperature dependence of an output voltage whensetting the control voltage of the current-voltage converter to 5 V inwhich the temperature dependence of the current-voltage converter isequal to that of the temperature sensor for the thin-film semiconductordevice of the structure of FIG. 5A of First Embodiment. The verticalaxis indicates the ratio of measured value at each temperature tomeasured value at 20° C. The characteristic of the control voltage of 10V used in the first embodiment is shown for comparison (-●-). Asindicated by -▪-, in the control voltage of 5 V, the output of thethin-film semiconductor device is almost constant without depending ontemperature. No outputs dependent on temperature can be obtained. Thefunction as the temperature sensor cannot be fulfilled.

Although the invention has been described in connection with severalpreferred embodiments thereof, it will be readily apparent to thoseskilled in the art that various modifications thereof are possiblewithout departing form the true scope and spirit of the invention as setforth in the appended claims.

1. A display comprising: a thin-film semiconductor device comprising: atemperature sensor formed of a thin-film semiconductor and sensing atemperature as current; and a current-voltage converter formed of athin-film semiconductor and having temperature dependence in which itscurrent-voltage characteristic is different from that of saidtemperature sensor; wherein a temperature sensed by said temperaturesensor is converted to a voltage by said current-voltage converter; animage display area; signal voltage driving circuitry driving saiddisplay part; reference voltage circuitry receiving an output voltage ofsaid current-voltage converter and determining a reference voltage givento said signal voltage driving circuitry; scanning electrode drivingcircuitry scanning the screen of said display image part; and controlcircuitry controlling said signal voltage driving circuitry and saidscanning electrode driving circuitry.
 2. The display according to claim1, wherein said image display area is a liquid crystal panel.
 3. Thedisplay according to claim 2, further comprising electric circuitryapplying an electric field larger than an ordinary video signal electricfield to said liquid crystal, wherein said electric circuitry iscontrolled by an output voltage of said thin-film semiconductor device.4. The display according to claim 1, further comprising a controllerchanging a reference voltage outputted from said reference voltagecircuitry by an output voltage of said current-voltage converter.
 5. Amethod of driving the display according to claim 1, comprising: changinga reference voltage outputted from said reference voltage circuitry byan output voltage of said current-voltage converter.